1. Field of the Invention
This invention relates to a clock signal fault detector for detecting extraordinary conditions of a clock signal used in respective controllers in a computer or the like.
2. Description of Related Art A known clock signal fault detector has a monostable multivibrator connected with a CR filter and detects whether a clock signal changes in a predetermined period determined by the time constant of the CR filter. The clock fault detector thereby judges that the clock signal is in the ordinary condition when one or more level changes of the clock signal have been detected within the predetermined period, and, determines that the clock signal is in an extraordinary condition when no level change of the clock signal has been detected within the predetermined period. The clock signal fault detector can therefore detect extraordinary conditions such that the clock signal is fixed, or stuck, at either a high level or a low level.
However, such a conventional technique as described above has raised the following problems. Where the clock signal is in the ordinary condition and repeats high and low levels alternately with a constant period, the clock signal is, with the conventional technique, assumed to be in the ordinary condition if at least one level change of the clock signal has been detected within the certain period of time. Accordingly, such a conventional technique could not distinguish a clock signal fault in which the clock signal changes between high and low levels with an extraordinarily high frequency, relative to the normal condition. Thus, the conventional technique could not detect this situation as an extraordinary condition.
With the conventional technique, the period of time to be monitored is determined by a time constant given by the capacitor and a resistor. Therefore, the monitoring period could not be set to a short time basis, such as a nanosecond basis, suitable for current clock precision. Since the conventional technique uses a clock signal fault detector, constituted of analog elements such as capacitors and resistors, the clock signal fault detector could not easily be incorporated into an LSI device.